Regulation of glycophorin gene expression by a tumor-promoting phorbol ester in human leukemic K562 cells.

We have previously shown that treatment of the human leukemic cell line K562 with the tumor promoter 12-O-tetradecanoyl phorbol 13-acetate (TPA) leads to a reduction in the expression of two surface glycoproteins, Gp-105 and glycophorin (Fukuda, M. (1981) Cancer Res. 41, 4621-4628). In this report, we examine the mechanism for reduction of glycophorin expression. By immunoprecipitation of metabolically labeled proteins, we found an 11-fold reduction in the incorporation of [3H]glucosamine into glycophorin after 48 h of TPA treatment. This was found to closely correlate with a reduction in [35S]methionine incorporation, suggesting that regulation occurs at the level of protein synthesis and not glycosylation. We also found that TPA decreased the incorporation of [3H]glucosamine into glycophorin in another leukemic cell line, HEL. A time course showed that there was a 3-fold reduction in glycophorin biosynthesis as early as 4 h after TPA treatment. The level then decreased to approximately 10% of the untreated control levels after 12 h of treatment. The reduction in glycophorin biosynthesis was found to be reversible following removal of TPA from the culture medium. By immunoprecipitation of in vitro translation products directed by purified total cellular RNA, we found that there is a corresponding decrease in glycophorin mRNA activity. Glycophorin mRNA activity was extensively reduced as early as 1 h after TPA treatment and by 12 h was nearly undetectable. Interestingly, the size of the primary translation product was found to be 8 kDa larger than the fully processed apoprotein. These results are consistent with the transcriptional regulation of glycophorin expression by TPA.